The present invention generally relates to metering devices for measuring the velocity of fluid flowing in a pipe, and is typically used to determine the flow volume rate in industrial applications, such as the transport of natural gas. More specifically, the invention relates to an optical system that focuses two light beams in the pipe through a transparent window in the pipe wall, and detecting the time of flight delay of light scattered by small particles carried by the fluid, as the particles pass from one focal spot to the other.
In pipeline operations and other industrial processes, flow meters are used to measure the flow rate of gases or fluids moving through the pipeline. There are many mechanical methods for determining the flow rate in pipes, including orifice plates, pitot tubes, Venturi meters, vortex meters, coriolis effect meters, variable area meters, and turbine meters, but generally they require that obstructive structures be inserted inside the pipe, which is undesirable in many applications because it disrupts the fluid flow and creates a pressure drop. Furthermore, many mechanical based sensors require that substantial gas pressures or flow rates be attained to produce a measurable effect. This is problematic for some applications where the reservoir pressure is very low, such as coal bed methane production, or when the fluid is vented to atmosphere or a large storage vessel.
Ultrasonic based meters are also known, which measure the Doppler shift of the acoustic velocity of ultrasound beams that are directed diagonally or along the pipe axis. Many ultrasound meters require pockets in the pipe walls to seat the ultrasound transducers, which is undesirable because contaminants tend to build up in the cavities. Long sections of pipe are required to accommodate the ultrasonic beam paths, which can be awkward and expensive, especially for large pipe diameters.
Other versions of ultrasonic flow meters launch the ultrasonic waves through the wall of the pipe, using clamp on transducers, but the accuracy performance suffers at low operating pressures and low flow rates.
Optical techniques for measuring the flow rate of fluids in pipes are also well known, and generally fall into two categories. Laser Doppler Anemometers use a single coherent laser that is split into two beams that are directed to intersect at the measurement point. The intersecting laser beams create an interference light pattern of alternating light and dark bands along the axis of the fluid flow. Particles passing through the measurement zone scatter the light, which creates a periodic varying optical signal, whose modulation frequency is proportional to the velocity of the particle. This technique is useful when measuring complex flows, where there are many large scattering particles, but because the light is distributed over many intensity maxima, the detection efficiency is low and small particles do not scatter enough light to be measured effectively.
The velocity of fluids can also be measured using a technique, generally referred to as the Laser-Two-Focus method. This system involves an optical delivery system that directs the light from one or two laser beams to form two focus spots in the pipe, separated by a known distance along the pipe axis. Particles in the fluid stream that pass through the two focus spots, scatter the light which is directed on to a photodetector by an optical collection system. The resulting signal consists of short impulses, and by measuring the time delay between adjacent pulses, the velocity of the particle can be determined. Because the intensity of the delivered light is concentrated in only two spots, the sensitivity of the Laser-Two-Focus method is superior to the Laser Doppler Anemometer system. This is important in certain fluids, such as natural gas, which contain only very small particles that are often less than 1 micron in diameter.
The amount of light scattered by a particle at a given angle depends on many variables, including the size, shape, surface quality, transparency/opacity, refractive index, and conductivity of the particles. The combination of these effects is very complex and generalized theories such as Mie and Rayleigh scattering fail to predict real world results accurately, so empirical studies are most often used to characterize specific systems. Mie theory is useful however in gaining a basic understanding of general trends in scattering behaviour. For instance it predicts that the amount of light scattered by very small particles (approaching the wavelength of the incident light or smaller), is subtended mostly within a very small, forward scatter angle. FIG. 1 shows that more than 90% of the light scattered by a spherical transparent droplet, with a diameter equal to 6 wavelengths of the incident light, occurs within a 10 degree forward angle cone.
The ability of a Laser Two Spot optical system to discriminate light scattered by a particle depends not only on the amount of light collected by the detection optics, but also by how much unscattered light is prevented from reaching the detector. For example, any light that is scattered at an angle less than the divergence cone of the incident light can not be effectively detected because the detector will be blinded by the unscattered light. The contrast or detectability of scattered light is fundamentally limited by the contrast ratio of detected scattered light to detected unscattered light.
Previous laser two spot optical flow meters, such as described by Kiel et al and Williamson et al, optimize the contrast of the detected light scatter signal, by shifting the optical axis of the collection optics away from the incident light axis, as shown in FIGS. 2a and 2b. This mininizes the signal bias caused by the unscattered light, but only a small amount of the scattered light is coupled into the collection aperture. In some cases, such as natural gas, where the size of naturally occurring scattering particles is very small, this can be a limiting factor and the signal to noise ratio will suffer due to weak detected scattered light levels.
Laser two spot anemometers are also known to characterize the flow of relatively large particles (greater than 10 wavelengths of the incident light) such as particulate dusts or aerosols. Hairston et al teaches a system for measuring the size and velocity of aerosols ejected by a nozzle, using the laser two spot method, with the collection aperture colinear with the incident beam axis. The unscattered light is blocked by a central obscuration located on the opposite side of the measurement zone, and light scattered at larger angles that pass into the collection aperture are focused on to a photo-detector. Because the particles are relatively large in this application, the detected light amplitude is not so much a concern, so a large central obscuration can be used without sacrificing sensitivity.
The optical systems described by Kiel, Williamson, Hairston et al., all feature telecentric or parallel optical systems, that generate delivery light beams that are directed perpendicular to the flow direction. This is important in some applications, particularly when the fluid is a gas under high pressure. Most low pressure gases have a refractive index vary near unity, but at high reservoir pressures, greater gas density causes a significant increase in the refractive index, which would change the optical refraction angle of any light passing into the medium. This can cause a parallax type shift in the spacing between the focus spots, if the optical axis is not perpendicular to the flow axis, resulting in a measurement error.
In some flow measurement environments, such as natural gas wells, a significant amount of water, liquid hydrocarbons, particulates, and other contaminants may deposit on the optical windows and degrade the efficiency of the transmitted light over time. This problem has not been effectively addressed in the prior art for applications where the optical metering apparatus is intended to be left in place for long periods of time. Optical windows are used in many other pipeline applications, particularly sight glasses, and there are a number of remedies that have been developed to allow for the windows to be cleaned from time to time. It is desirable however, to develop an optical system that both resists fouling and is tolerant of variations in the optical transmission efficiency.
Also, for many industrial applications, information on size and shape of the particles flowing in the pipe is highly desirable to characterize and monitor the quality of the fluid in the process. This data verifies, for example, the quality of filtering means used at a natural gas processing plant, condition of the pumps and corrosion level of the pipes. Specialized laser devices for measuring particle size are known, but their use has been largely limited to controlled laboratory environments and they are not considered suitable for in-field applications due to their sensitivity to vibration and misalignments.